This invention relates to a method of minimizing the carryover of slag during the draining of molten metal from a ladle or other vessel. It relates particularly to a method of preventing a significant carryover of slag when molten steel is drained from a ladle or tundish into a continuous casting machine or into ingot molds.
When molten steel has been suitably refined in a furnace and is ready to be cast, the molten steel is tapped or poured into a refractory lined steel transfer ladle. The molten steel often then has further treatment, such as desulfurization, while in the ladle before the ladle is taken to a continuous casting machine or an ingot teeming station.
When the molten steel is tapped from the refining furnace into the ladle, a certain amount of the furnace slag is carried over with the molten steel into the ladle. In addition, while the molten steel is in the ladle, slag forming materials are often added to assist in the further treatment of the steel while in the ladle or to act as an insulation.
Since the slag is less dense than the molten steel, the slag will float on the surface of the molten steel contained in the ladle. However, as the steel is drained from the ladle through an orifice in the bottom of the ladle, some slag will carryover into the tundish of the continuous casting machine or into the ingot mold unless the operator is very careful. Slag in the tundish or the ingot molds results in a poor quality cast steel product. As a result, it is common for operators to leave a substantial amount of steel in the ladle to avoid any slag carryover. This practice results in a poor yield to the steelmaker.
In recent years a number of fabricated plugs or stoppers of a density between that of the molten steel and the slag in the ladle have been developed and patented. These fabricated plugs and stoppers are designed to float at the slag-molten metal interface directly above the drain orifice in the ladle and are drawn into the orifice to prevent the entry of slag as the molten metal drains out of the ladle. The following United States patents describe the various shapes and configurations for these fabricated plugs or stoppers.
______________________________________ U.S. Pat. No. ______________________________________ 2,246,144 Perrin 1941 Raft 2,718,389 Perrin 1955 Dam 4,462,574 Keenan 1984 Cube 4,494,734 LaBate 1985 Rod Stopper 4,526,349 Schwer 1985 Disc 4,601,415 Koffron 1986 Tapered Polygon 4,610,436 LaBate 1986 Rod Stopper 4,709,903 LaBate 1987 Rod Stopper 4,725,045 Cutre 1988 Cone 4,799,650 LaBate 1989 Rod Stopper ______________________________________
These fabricated plugs or stoppers are expensive and also require a precise placement of the device right over the drain orifice to be effective. Most require an elaborate boom or mechanical arm to reach into the ladle or vessel and position the plug or stopper right over the drain orifice. If such devices are not accurately positioned, they are ineffective.